loud bang when boiler fires up, fault code F1 if it fails to fire?

Glow Worm flexicom 30 SX
Our boiler is just over two years old. It often fires up totally normally but sometimes this is preceeded by a loud bang through the flu duct presumably as the gas ignites, although a local boiler engineer thought the bang was before the flame ignited. Is this possible. If it fails to light, then a fault code F1 is displayed. Pressing the reset button clears this and the boiler will then ignite either with or without the loud bang.